In recent years there has been an increasing interest in the development of the world market for monomers of .alpha.,.beta.-unsaturated carboxylic acids in general, and acrylic acid in particular. Much of this interest reflects the importance of useful polymers which can be produced from these .alpha.,.beta.-unsaturated carboxylic acids for products such as paints, coatings, lacquers, water treatment, secondary oil recovery, and superabsorbent polymers.
Although acrylate esters are currently stored and transported on a routine basis, it is well recognized that acrylic acid is significantly more reactive than acrylic esters and the same methods employed for the storage and transport of the esters are not applicable to the acid. Acrylic acid is one of the most reactive of the common vinyl monomers. Due to his reactivity, glacial acrylic acid in particular has only been marine shipped in bulk (i.e., in a ship's hold tank) once. In the 1970's, Union Carbide attempted to ship a cargo of glacial acrylic acid across the Atlantic in the ship Alchemist. There was a runaway polymerization at sea and the ship was seriously damaged (Kurland, J. J., and Bryant, E. R. Plant/Operations Progress 6(4), 203-207 (October, 1987)). Since then, to the present inventor's knowledge, no one has attempted a bulk marine shipment of glacial acrylic acid. Acrylic acid, however, inhibited with .gtoreq.100 ppm phenothiazine (PTZ) is currently shipped over land and by marine shipment.
The risk of bulk marine shipments of glacial acrylic acid has been found to be unacceptably high. This is due, in part, to the limitations on safety procedures which can be implemented should a runaway polymerization occur at sea. On land, emergency personnel can respond to such an incident and evacuations may be conducted if necessary. At sea, these procedures are much more difficult, if not impossible, placing the crew at increased risk should a runaway polymerization occur. Another risk factor for marine shipment is that, in general, longer time periods are required for the transport than for land shipment. This further increases the risk of a runaway polymerization.
Inhibitors of polymerization include PTZ and phenolic inhibitors such as hydroquinone (HQ) and p-methoxyphenol, which is the monomethyl ether of hydroquinone (MEHQ). While these phenolic inhibitors are effective for acrylic acid significantly more of the phenolic inhibitor is required to achieve the same level of inhibition as PTZ. The phenolic inhibitors also require dissolved oxygen in order to be effective. This requirement could limit their use for storage and transport due to the possibility of inadvertent heating or the presence of unexpected polymerization inducers which could exhaust the available oxygen and consequently lead to runaway polymerization.
Published JP patent application No. 51-98211/1976 (21117/1975) relates to a method of preventing polymerization of an .alpha.,.beta.-unsaturated carboxylic acid by addition of certain manganese salts. Generally, however, such materials are not useable because they are too unreactive. JP 51-98211 does not teach removal of the Mn to allow usage of the acid.
Japanese published application 5-140027, (application no. 4-133351) relates to a method of inhibiting polymerization of an .alpha.,.beta.-unsaturated carboxylic acid by addition of a threefold system including phenol, hindered phenol, and Mn salt.
Published JP application no. 08-310979 (JP 8310979) relates to purification of a vinyl monomer by removing polymerization inhibitors with a certain adsorbent such as alumina, silica gel, molecular sieve, activated carbon, ion exchange resin, chelate resin, zeolite, and acid clay.
JP publication 06-211735 (JP 6211735A) relates to producing methacrylic acid and prevention of polymerization of methacrylic acid during transportation and storage.
JP publication 05-017512 (JP 5017512A) relates to prevention of polymerization of an .alpha.,.beta.-unsaturated carboxylic acid by addition of p-phenylenediamine derivative and a manganese compound.
EP 371748 relates to inhibition of polymerization during distillation of monomers by means of a phenolic inhibitor and a manganese or cerium alkanoate of certain monocarboxylic acid.
US patents describing Mn and/or Ce as inhibitors for ester production wherein acrylic acid is employed as starting material are U.S. Pat. No. 4,814,493, 4,507,495, and 4,542,231. These patents do not address storage and transport of or removal of metals in acrylic acid.
Because of the risk factors regarding polymerization of .alpha.,.beta.-unsaturated carboxylic acid particularly for storage and shipping in bulk, and the need for desired grade product after storage and shipment, known procedures are not completely satisfactory.